Certain towing vehicles (e.g., such as used in agriculture or forestry in the form of agricultural traction engines, tractors, and combine harvesters; and, also light trucks) have hydraulic brake systems that, as a rule, consist of brake cylinders that are actuated by foot brake pedals and that feed hydraulic fluid to the corresponding wheel brake cylinders of the towing vehicles' wheels. In contrast, the trailers that can be coupled to these towing vehicles are pneumatically braked. On account of the different pressure media used—namely, hydraulic fluid for the service brake system of the towing vehicle and compressed air for the brake system of the trailer vehicle—it is necessary in vehicle combinations of this type to convert the hydraulic brake pressure of the towing vehicle into a pneumatic pressure for the brake system of the trailer. A trailer control valve that is arranged on the towing vehicle (e.g., WABCO GmbH part number 470 015), has been used for decades to this end. Such trailer control valves are of purely mechanical construction; in them, hydraulic control pressure is converted into pneumatic output pressure and is fed to the trailer vehicle.
A disadvantage of a purely mechanical trailer control valve is that, on account of relatively large hysteresis, feeding brake pressure to the trailer vehicle can involve a certain delay, which might lead to restricted modulation ability and poor response behavior. One advantage of the conventionally constructed trailer control valve is its robust design, which is relatively insusceptible to disruptions. Because such a trailer control valve is operated purely mechanically, no electronic component is present that can fail.
Applicant's DE 197 52 147 B4 describes a brake system for a hydraulically braked motor vehicle, which can be coupled to a pneumatically braked trailer, in which the hydraulic pressure of the service brake is utilized only indirectly to control the trailer's pneumatic brake system. The hydraulic pressure of the outlet lines of the brake master cylinder is converted by a pressure sensor into an electrical signal, which is fed to an electronic control device that evaluates the pressure signal and switches a proportional relay valve, which is connected between a compressed air reservoir and a purely pneumatically controlled trailer control valve. The proportional relay valve is normally closed and is opened only when a corresponding signal is applied to it by the control unit. One advantage that very rapid switching times are made possible, since the problem of hysteresis associated with conventional trailer control valves is dispensed with. However, in the redundancy case that is, in the event of a partial or complete failure of the electronics), solenoid valves, for example, are no longer switched (no longer opened or closed). This can lead to a situation where either braking can no longer be carried out, or an emergency braking operation is effected for instance, by the full supply pressure being guided through a normally open solenoid valve and being fed to a brake system.
EP 2 165 901 131 is concerned with the redundancy case. An electropneumatic loading path and a hydropneumatic loading path are provided. In the normal case, that is, when there are no electrical or electronic faults, a corresponding pressure signal in accordance with the hydraulic pressure is fed to a pneumatic pressure regulating module that generates a pneumatic output pressure for the pneumatic trailer brake system depending on the electrical signal it terms of its function, the pressure regulating module corresponds to an electropneumatic trailer control valve of a truck brake system, in which both the owing vehicle and the trailer vehicle are braked pneumatically. A conventional trailer control valve, in which hydraulic control pressure is converted into a pneumatic output pressure, is connected upstream of the pressure regulating module. The pneumatic output pressure of the trailer control valve is used as control pressure for a control inlet of the pressure regulating module. In the redundancy case, that is, in the case of a failure of the pressure regulating module, the pneumatic output pressure of the trailer control valve is guided through the pressure regulating module and is fed to the trailer brake system. This provides a fallback option in which the trailer brake system can still be actuated. However, disadvantageously, the hydropneumatic trailer control valve is connected upstream of the electropneumatic pressure regulating module. The pressure regulating module therefore receives its pneumatic control pressure from the trailer control valve even in the case of a fully functional electronic regulator. Here, the disadvantages of the hydropneumatic trailer control valve come into effect, since, as a result of the hysteresis of a valve arrangement of this type, the control pressure for the pressure regulating module is supplied with a certain delay, as a result of which the advantages of an electropneumatic pressure regulating module are eliminated at least partially.